Methods for Providing a Durable Solar Powered Aircraft with a Variable Geometry Wing

ABSTRACT

Methods of manufacturing and operating a solar powered aircraft having segmented wings that can be reconfigured during flight to optimize collection of solar energy are described. The aircraft have rigid construction that is resistant to inclement weather and is configured to rely on free flight control at high altitude and under conventional conditions, thereby providing flight duration in excess of 2 months. The aircraft is particularly suitable for use as part of a telecommunications network.

This application claims the benefit of U.S. Provisional Application No.62/152,747, filed Apr. 24, 2015, U.S. Provisional Patent Application No.62/120369, filed Feb. 24, 2015, and U.S. Provisional Patent ApplicationNo. 62/120,361, filed Feb. 24, 2015. These and all other referencedextrinsic materials are incorporated herein by reference in theirentirety. Where a definition or use of a term in a reference that isincorporated by reference is inconsistent or contrary to the definitionof that term provided herein, the definition of that term providedherein is deemed to be controlling.

FIELD OF THE INVENTION

The field of the invention is aircraft, particularly solar poweredaircraft.

BACKGROUND

Orbiting and geosynchronous satellites are in wide use for communicationbetween ground communications hubs and directly between communicationusers. The revenue of the satellite industry exceeded $200 B in 2014,and over 400 satellites are currently in geosynchronous orbits.

The preparation and delivery costs of such satellites represents aconsiderable expense, however, and rocket launches themselves representa non-negligible risk. While satellites have the advantage of requiringno significant energy in order to remain in orbit for years, they lackthe ability to recover and to change orbits as may be required to modifythe communication equipment or the serviced area. Additionally, whilethe satellites offer wide area coverage their great communicationsdistance results in the need to use high transmission power andrelatively narrow antenna apertures, which in turn results incommunications time delays.

The idea of sub-orbital vehicles as airborne communications nodesutilizing both lighter than air and heavier than air vehicles is wellknown. For example, the military use communication relay aircraft inspecific applications where they offer a versatility advantage comparedto satellites.

Because of their required high launch energy and extreme systemreliability, satellites present substantially higher initial costs thanaircraft. However, by maintaining orbit for extended periods of timethey can offer lower cost per hour. In order to compete effectively withsatellites, an aircraft needs to provide large communications areacoverage, dependable service, and many hours of operations per launchand recovery cycle. To provide large area coverage for high frequencyband line-of-sight communications and for dependable service, such anaircraft must fly at high altitudes and above weather. For very longendurance the aircraft will preferably be unmanned and use a renewableenergy source. When flying above the weather, solar energy is a readilyavailable energy source for aircraft with endurance greater than a fewdays. Solar powered unmanned aircraft first flew in 1974, and mannedaircraft first flew in 1979. Solar energy, however, is a relativelydiffuse source that can limit the power available to propel theaircraft. In addition, reliance on solar power can restrict the latitudeand/or time of year that such aircraft can operate effectively.

Flying at high altitude at low power levels requires a very lightaircraft with a large lifting surface area (i.e. low wing loading).Unmanned solar powered aircraft developed between 1983 and 2003 byAeroVironment, Inc. under NASA's Environmental Research Aircraft andSensor Technology (ERAST) program aimed at ever increasing aircraftcruise altitude. On Aug. 14, 2001 their Helios unmanned aircraft set analtitude record of 96,863 feet (29,524 m). To achieve such a highaltitude the highly innovative Helios design included a wing span of 247feet, a wing area of 1,976 square feet, and a normal weight of only1,600 lb (0.81 lb/ft² wing loading). In order to achieve such a lowweight, Helios used an all-wing design, a very light flexible structure,a high level of weight distribution along the span (span-loading), and adistributed powerplant configuration. Unfortunately, such extremelyflexible structures are relatively fragile. The Helios was lost in aninflight failure, with aircraft loss due to wind gusts on Jun. 26, 2003.

Flying in clear weather or at low altitude is less challenging, whileless useful for communications purposes, has provided progress indeveloping the system reliability required for long endurance flight.For example, QinetiQ's 74 ft span Zephyr unmanned solar powered aircraftset, on July 2010, a 336 hour, 22 minute endurance record for unmannedaircraft. While the Zephyr endurance record was set during summer months(from July 9 to July 23 in Yuma, Ariz., 32 degrees north latitude)), inlater tests Zephyr flew 11 days in winter conditions.

Long duration solar powered aircraft are also hampered by the relativelylow energy density of regenerative batteries and regenerative fuel cellsrequired to power the craft at night. Flying at high altitude isadditionally challenging in this regard, as the low air densityincreases the power required for propeller driven aircraft (e.g. by afactor of 3× relative to sea level at 56,000 ft and a factor of 4×relative to sea level at 69,000 ft). Additionally, the increased dynamicviscosity at high altitude (resulting in reduced Reynolds numbers)results in reduced aerodynamic performance in terms of aircraftlift/drag ratio and propeller efficiency for slow flying aircraft.

We assume that in order for the high altitude solar powered unmannedaircraft to become a desirable alternative to the low-orbit satelliteand to capture a portion of the large communication market it needs tooffer:

-   -   a. a network of hundreds of aircraft on station worldwide        providing continuous coverage    -   b. an airframe of adequate ruggedness to withstand climb and        descent in less than ideal weather    -   c. a flight safety track-record documenting an ability to        sustain approximately 9,000 flight hours per year per aircraft,        using aircraft certified by the Federal Aviation Administration        (FAA) and of other world agencies and the approval of the Air        Traffic Control (ATC)    -   d. sufficient geographic coverage during winter    -   e. a cost per communication data rate and per area coverage that        is reasonably competitive with geosynchronous satellites.        Providing such a combination of essential aircraft attributes        requires that a large and very light aircraft achieve a safety        record comparable to that of current commercial airlines. While        many individual technology advancements (e.g. improved battery        energy density and/or solar cell efficiency) can contribute to        this, it is important to combine the performance of all        contributors.

A key challenge for a solar powered aircraft to provide an acceptablemarket entry is the ability of the aircraft to collect solar energy atlow sun angles, such as in winter and early and late in the day. Priorart aircraft configurations typically utilize a flat, stretched wing,which offers high aircraft lift/drag (L/D) ratio and low powerrequirements. The challenge of relying on solar energy is generallyaddressed by: a) building ultra-light, low wing loading airframes thatare not capable of surviving gusts, b) flying in mid-summer (when highsun angle provides optimal solar cell performance), c) flying ingeographic locations where sun angle is high (e.g., low latitudes), d)flying at low altitude and in good weather (permitting lower speed andreduced power requirements), e) not flying through the night, and f)gliding at night using altitude gained during the day (with resultingloss of altitude), and frequently utilize a combination of these. Suchapproaches, however, are not viable for the intended use of replacingorbiting communications satellites.

The challenge of flight through the night can be partially addressed byuse of high energy density batteries (for example, Lithium-Sulfurbatteries) with energy densities of up to 220 Wh/lb. Laboratory researchindicates further improvement in performance of several chemistries ofrechargeable batteries are possible.

Some solar powered aircraft have attempted to address the problem ofgenerating adequate power at low sun angles by presenting solar cells atangles that are more vertical relative to the plane of the aircraft whenin level flight. U.S. Pat. No. 6,931,247 (to Cox and Swanson) disclosesa lightweight solar powered aircraft configured as a flying wing,constructed as a transparent film over a lightweight frame. The wing isdivided in segments that are joined by hinges, with each segmentcarrying motor/propeller assemblies. The hinges permit adjustment of thedihedral angles of the segments of the wing that aid in orienting solarcells mounted on the wing toward the horizon during flight. Hingeplacement and orientation permit various wing configurations, including“M” and “W” configurations with both positive and negative dihedralangles. All publications identified herein are incorporated by referenceto the same extent as if each individual publication or patentapplication were specifically and individually indicated to beincorporated by reference. Where a definition or use of a term in anincorporated reference is inconsistent or contrary to the definition ofthat term provided herein, the definition of that term provided hereinapplies and the definition of that term in the reference does not apply.United States Patent Application Publication No. 2010/0213309 (to Parks)shows a similar lightweight aircraft, which includes tail booms thatcarry additional solar cells and wing-mounted pylons that positionmotor/propeller assemblies both above and below the wing. Differentialthrust applied by these motor/propeller assemblies is used to adjust thepitch of the aircraft for altitude adjustment. The taught aircraft,however, utilize motive power (in the form of motor/propellerassemblies) in each wing segment, apparently as a design constraintnecessitated by flexibility of the taught airframes. This increasesexpense and design complexity, and provides numerous opportunities forcomponent failure.

Another approach that has been attempted to improve the efficiency ofsolar cell performance in aircraft flying at low sun angles is toprovide a vertical surface on which solar cells are mounted. In suchdesigns, however, the aircraft L/D ratio is necessarily reduced inproportion to the improved low sun angle collection. This results fromthe additional parasitic drag of vertical surfaces beyond thosenecessary for efficient flight, resulting in increased powerrequirements. In addition, such vertical surfaces can negatively impactaircraft stability in inclement weather.

Still another approach that has been attempted in the prior art is toutilize a flexible wing surface that carries solar cells. When thecentral portion of the wing is weighted (for example, through carryingbatteries or payload) the downwards inflection of the flexible wingproduces a curve that angles a portion of the solar cells towards thehorizon. An example of such a design is the aforementioned Heliosaircraft. Bending of the aircraft's flexible wing combined with abuilt-in 5 degrees dihedral of the outboard wing sections providedadequate solar energy collection during flight at the relatively highsun angles provided by performing the flight test in August at a lowlatitude (i.e. Kauai, which lies at 22° N latitude). Unfortunately, asdemonstrated by the loss of Helios aircraft due to weather, such aflexible wing does not provide an aircraft that is sufficiently ruggedfor long duration flight.

Thus, there is still a need for solar powered aircraft that can provideconsistent long duration flight times at high altitudes and highlatitudes.

SUMMARY OF THE INVENTION

The inventive subject matter provides apparatus, systems and methodsthat provide a solar powered aircraft capable of long duration flightsat high altitude and in latitudes greater than 30° N and 20° S. Theaircraft is of rigid, molded construction, and includes a segmentedwing. Individual wing segments are joined by hinges adjustable topositive and negative dihedral angles during flight. This permitsorientation of photovoltaic cells towards the horizon when the sun is ata low angle, thereby increasing the efficiency of the photovoltaiccells. The aircraft's molded construction provides sufficient rigidityto permit the aircraft to fly through inclement weather conditions.Reliability and safety in long duration flights is further increased byreliance on free flight (with no active control) for greater than 90% ofthe flight, which reduces wear on active control surfaces. Aircraft ofthe inventive concept are capable of flight durations of 2 months to 5years at altitudes in excess of 50,000 feet and at latitudes between 40°N BS 30° S.

Such an aircraft can include an on-board controller that is programmedto provide autonomous flight of the aircraft under both slow response(such as a roll trim function) and rapid response if required forstabilizing the aircraft in gusts. Slow response conditions includesteady flight under normal weather conditions at cruising altitude. Freeflight is utilized under slow response conditions, where free flightcontrol may not utilize control surfaces but rather utilizes changes inpropeller speed to control the aircraft. Rapid response conditionsinclude takeoff, landing, inclement weather, and rapid coursecorrections transmitted from the on-board controller or a remoteoperator. Active flight control utilizes control surfaces that arecoupled to actuators. The controller is programmed to optimize the useof active control to minimize the duty cycle of the actuators, therebyreducing the chances of mechanical failure. The controller can utilizesensor data and/or input from an external source to determine if slowresponse or rapid response conditions exist.

Various objects, features, aspects and advantages of the inventivesubject matter will become more apparent from the following detaileddescription of preferred embodiments, along with the accompanyingdrawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts solar energy availability at different latitudesthroughout the year.

FIG. 2A depicts solar elevation at different times of the day at 50° Nlatitude at different times of the year.

FIG. 2B depicts maximum seasonal wind speeds at different longitudes andlatitudes.

FIG. 3 shows the relationship between drag and Reynolds number (Re) foran airfoil.

FIGS. 4A to 4C depict an embodiment of an aircraft of the inventiveconcept that includes a boom-mounted tail. FIG. 4A shows the aircraftwith the wing segments held in an essentially horizontal position. FIG.4B shows the aircraft with the wing segments held in a partially flexedposition to provide dihedral and anhedral angles for increasedefficiency of solar energy collection. FIG. 4C shows the aircraft withthe wing segments held at a greater degree of flexion.

FIGS. 5A and 5B depict a flying wing embodiment of an aircraft of theinventive concept (i.e. one that does not include a tail). FIG. 5A showsan orthogonal view of the aircraft. FIG. 5B shows a top-down view of theaircraft, showing a swept wing design.

FIGS. 5C and 5D depict hinges utilized in joining wing segments. FIG. 5Cdepicts a hinge joining an inboard lateral wing segment and an outboardlateral wing segment, and includes an inset showing the position of thehinge when the wing is angled. FIG. 5D depicts a hinge joining aninboard lateral wing segment to the central segment of the aircraft.

FIGS. 6A and 6B depict control surfaces utilized by an aircraft of theinventive concept. FIG. 6A shows various positions of a simple controlsurface associated with an airfoil and coupled to an actuator. FIG. 6Bshows various positions of a split control surface associated with anairfoil and coupled to an actuator.

FIG. 7 depicts a cross section of a wing segment of an aircraft of theinventive concept.

FIG. 8 depicts mass distribution across an aircraft as depicted in FIG.5A.

FIGS. 9A and 9B depict stiffness of a wing of an aircraft as depicted inFIG. 4A. FIG. 9A shows bending stiffness. FIG. 9B shows torsionalstiffness.

DETAILED DESCRIPTION

The following description includes information that may be useful inunderstanding the present invention. It is not an admission that any ofthe information provided herein is prior art or relevant to thepresently claimed invention, or that any publication specifically orimplicitly referenced is prior art.

High altitude, long flight endurance aircraft present an attractivealternative to satellites in telecommunications networks. One obstacleto the development of these is a power source that can sustainuninterrupted flight for weeks, months, or years without the need foractive refueling. Solar power provides a potential solution, butpresents challenges beyond the obvious day/night cycle. As shown in FIG.1 the solar energy available in kilowatt per square meter varies as afunction of time of year and geographical elevation (i.e. latitude). Itis apparent that in the stratosphere (i.e. above the weather)significant solar energy is available in mid-winter at elevations of39.3 degrees north (4 kWh per day per square meter over Stuttgart) andeven at 48.8 degrees (2.5 kWh per day per square meter over Berlin).However, photovoltaic cells provide optimal performance when the sunangle is essentially perpendicular to the photovoltaic cell's surface.As shown in FIG. 2A sun angles are shallow early and late in the day.This is especially true in winter at latitudes north of 35° N latitudeand south of 35° S latitude, which renders solar cells on horizontalsurfaces (such as aircraft wings) ineffective for energy collection.

Another challenge to the viability of solar powered aircraft is wind atthe intended operational altitudes. Even at the relatively low windspeeds experienced at 65,000 ft, the wind over many geographicallocations of interest for telecommunications relay can be greater thanthe flight speed of conventional current solar powered aircraft at thataltitude. Additionally, it should be appreciated that aircraft speed canbe required to be significantly higher than wind speed in order to beable to direct the aircraft's solar panels toward the sun.

Aircraft speed for best lift/drag ratio (i.e. the best range speed forpropeller-driven aircraft) is proportional to the square root of theaircraft's wing loading (aircraft weight divided by wing area). Theaircraft speed for minimum cruise power, which is preferred for bestendurance, is lower than that for best range. In order to achieve flightat high altitude, prior art solar powered unmanned aircraft minimize thepower required for flight by being very slow, very light, and having alarge wing area that results in low wing loading and low speed ataltitude. The wing loadings of the most successful prior art solarpowered aircraft are 0.22 lb/ft2 (psf) for the QinetiQ (now Airbus)Zephyr 6, 0.36 psf for Zephyr 7, and 0.81 psf for the AeroVironmentHelios, which produced estimated speeds at 70,000 feet of 30 knots, 50knots and 70 knots respectively.

Advances in photovoltaic cell efficiency and energy storage deviceperformance can offset these issues to some extent. However, even withbenefits of the rapidly advancing technologies of regenerative batteriesin terms of energy density (kWh/Lb at long cycle life) and of solarcells in terms of collection efficiency (percent of solar energyconverted to electric power), conflicts persist between the designconstraints of a payload-carrying aircraft and the use of solar power.Examples of such conflicts include:

-   -   a. Minimizing battery and/or fuel cell weight fraction (battery        weight and cells weight divided by total aircraft weight) while        maximizing energy storage capacity.    -   b. High cruise speed at high altitude is required to maintain        position against high winds, which demands high cruise power        (high propulsion system weight) and/or a combination of high        wing loading and high L/D ratio, while minimizing aircraft        weight is desirable.    -   c. Reliable and safe ascent and descent of the aircraft in        non-ideal weather requires a more robust airframes and higher        wing loading than what has been achieved in prior art high        altitude solar powered aircraft and can require measures to        reduce wing bending in gusty weather that are not accommodated        by traditional light weight structural components.    -   d. At a given aircraft weight higher wing loading is achieved by        reducing the wing area, which both reduces the area available        for solar cells and reduces the glide ratio (L/D) and results in        increased cruise power requirements    -   e. At a given altitude and energy storage density the endurance        of aircraft cruise through the night using only stored power        (which defines the maximum available operational latitude in        winter) depends on achieving low cruise power at night and high        collection of energy during the day (in excess over that needed        for day cruise), both of which require a large wing and low wing        loading.    -   f. High aircraft L/D ratio is best achieved with a flat wing (no        dihedral or anhedral) having a high aspect ratio (ratio of span        to average chord) and a large wing with a small fuselage and        tail similar to the configuration of competition sailplanes.        Power collection at low sun angle, however, requires solar cell        orientation towards the sun at low angles and therefore a        combination of dihedral, anhedral and vertical surfaces all of        which result in high parasite or induced drag and in lower L/D        ratio and higher required power for cruise at night.

Embodiments of the inventive concept include autonomous solar poweredaircraft configured for long endurance (i.e. greater than 2 months)flight at high altitudes (greater than 50,000 feet) at high latitudes(i.e. greater than 20° N and 20° S latitude). The aircraft is wellsuited for carrying a payload of telecommunications equipment (forexample, transmitters and/or receivers) and can act as atelecommunications relay within a telecommunications network when soequipped. Unlike prior art solar powered aircraft. Aircraft of theinventive concept are of rigid construction that is resistant todeformation under stress. Such aircraft can include a swept liftingsurface, for example a swept wing. The inventors have that such a sweptlifting surface is effective in controlling pitch and damping pitchoscillations (and thereby passively improving aircraft stability). Thewing of the aircraft supports photovoltaic cells for power generation,and is segmented. Hinges between the wing segments permit positive andnegative dihedral adjustment that permits orientation of photovoltaiccells towards the horizon for effective collection of solar energy atlow sun angles.

The following discussion provides many example embodiments of theinventive subject matter. Although each embodiment represents a singlecombination of inventive elements, the inventive subject matter isconsidered to include all possible combinations of the disclosedelements. Thus if one embodiment comprises elements A, B, and C, and asecond embodiment comprises elements B and D, then the inventive subjectmatter is also considered to include other remaining combinations of A,B, C, or D, even if not explicitly disclosed.

As used herein, and unless the context dictates otherwise, the term“coupled to” is intended to include both direct coupling (in which twoelements that are coupled to each other contact each other) and indirectcoupling (in which at least one additional element is located betweenthe two elements). Therefore, the terms “coupled to” and “coupled with”are used synonymously.

Unless the context dictates the contrary, all ranges set forth hereinshould be interpreted as being inclusive of their endpoints, andopen-ended ranges should be interpreted to include only commerciallypractical values. Similarly, all lists of values should be considered asinclusive of intermediate values unless the context indicates thecontrary. The recitation of ranges of values herein is merely intendedto serve as a shorthand method of referring individually to eachseparate value falling within the range. Unless otherwise indicatedherein, each individual value with a range is incorporated into thespecification as if it were individually recited herein.

As noted above, unlike prior art designs the aircraft is of rigidconstruction, with flight surfaces constructed from rigid molded and/orlaminated materials. Such an aircraft can include active controlsurfaces, and can include a controller that utilizes free flight controlunder slow response conditions (such as cruising at altitude under calmweather conditions and/or directed gradual course changes) and activecontrol utilizing actuator-driven control surfaces under rapid responseconditions (such as takeoff, landing, inclement weather, and directedrapid course changes). The controller can adjust the dihedral angle ofthe wing segments, and can direct active control surfaces via actuators.The aircraft can be inherently stable and the controller programmed tominimize the use of active control surfaces in order to minimize wearand improve safety and reliability in long flight duration.

It should be appreciated that designers of solar powered aircraft facedesign conflicts between the wing geometry needed for efficientcollection of solar energy, the wing geometry needed for efficientaerodynamic flight, the need for low aircraft weight, structuralrobustness required for tolerance of weather, and system robustnessrequired for extended flight times. Nearly all known solar poweredaircraft utilize the large, essentially flat surface of the main wing asa mounting site for large-area solar panels used to gather energy forflight. A conflict arises between the different orientations requiredfor this large surface to function in its dual roles as energy collectorand lift generator. Efficient collection of solar energy requires solarpanels oriented in a direction as nearly normal as possible to thedirection of incoming sunlight. When the sun is low in the sky (as isthe case at high latitudes, especially in winter, and is the case bothin the morning and evening at all latitudes and seasons) this requireslarge surfaces held in a close-to-vertical orientation.

Conversely, the efficient aerodynamic performance necessitated by therelatively low energy density provided by sunlight and the limitedcollection area requires a broad wing span, oriented normal to the localdirection of gravity (i.e. parallel to the horizon). Except when the sunis very high in the sky, these two requirements are conflicting. Thepresent invention resolves this conflict by providing a hinged wingsurface which is able to adapt the orientation of its surfaces dependingon operating conditions. During night-time hours, or during daylighthours when the sun is high in the sky, the wing is configured as a flat,horizontal, broad-span, efficient lifting wing. When the sun is low inthe sky, portions of the wing fold to orient larger panels more normalto the sun. In such conditions, the folded wing is less aerodynamicallyefficient, but the augmented efficiency in solar collection capabilityresults in a positive total change in excess power available. Inaddition, the disclosed designs provide a robust airframe that supportscentral placement of a single or small number of motor/propellerassemblies (thereby simplifying design and reducing the opportunitiesfor component failure) while providing the aircraft with the ability towithstand prevailing winds and inclement weather. As shown in FIG. 2B,which illustrates typical seasonal maximum wind speeds at differentlatitudes, prevailing wind speeds can increase with increasing latitudeand present a further limitation to the use of conventional ‘gossamer’construction of solar powered aircraft in such areas.

All known solar powered airplanes are limited in the northern andsouthern latitudes at which they can operate, especially during wintermonths when daylight is available for significantly less than half ofthe day and the sun remains at a low throughout. These limitationsresult from the inability of horizontally-oriented solar panels tocollect sufficient energy due to highly sub-optimal orientation forefficient collection. The present invention enables solar powered flightat higher latitudes than previously possible (for example, up to and/orbeyond 40° N latitude and 30° S latitude) by providing an adaptable wingthat is able to both fly with maximum aerodynamic efficiency during thelonger night time hours, as well as take best advantage of theavailability of sunlight during the limited hours when it is available.

Solar powered aircraft of the inventive concept include one or moreenergy storage devices that can store excess energy gathered duringdaylight hours for use under low-light conditions. Such energy storagedevices preferably are light weight, have high energy storage capacity,and are capable of sufficient charge/discharge cycles to sustain flightdurations of months to years. Suitable energy storage devices includerechargeable batteries, fuel cells (for example, a hydrogen fuel cellfueled by products of electrolysis), and/or capacitor banks. An aircraftof the inventive concept can use more than type of energy storagedevice.

In some embodiments of the inventive concept the wing is a swept-wingdesign, positioned at an angle relative to the direction of travel. Insome embodiments the aircraft is a ‘flying wing’ design, lacking a tail.Alternatively, in some embodiments the aircraft includes a tail whichcan serve, at least in part, as a lifting surface. Generally such sweptdesigns are considered unnecessary for low speed flight. In an all-wingdesigns swept wings improve pitch control effectiveness and damping ofpitch oscillations that reduces the need to rely on active controlsurfaces (thereby increasing aircraft safety and reliability in longendurance flights) while still providing adequate lift at acceptably lowaircraft weight. In some embodiments the sweep angle is about 5°, about7.5°, about 10°, about 12.5°, about 15°, about 17.5°, about 20°, orgreater than 20°.

Another design constraint of high altitude long endurance solar poweredaircraft is weight. The low weight requirements of solar poweredaircraft have resulted in prior art designs that sacrifice structuralstiffness and strength for weight. For example, prior art solar poweredHigh Altitude Long Endurance (HALE) aircraft are generally gossamer(i.e. very light and delicate) in nature, and are constructed fromlightweight polymer film stretched over minimal lightweight frameworks.Some of the most successful were developed by AeroVironment, Inc., whichalso developed the record breaking and highly publicized man-poweredaircraft Gossamer Condor and Gossamer Albatross. Such solar powered HALEaircraft are typically launched and recovered at carefully selectedtimes and locations in order to assure near-perfect calm weather.Unfortunately, weather prediction is often not adequately precise. Forexample, the solar powered Helios was lost due to gusty weather, whichresulted in deformation of the aircraft that ultimately led tostructural failure. The fragility of solar powered HALE aircraft is aresult of at least the following design and operational constraints:

-   -   a. very low structural weight fraction (weight of structure        divided by gross weight)    -   b. very low structural wing loading (weight of structure divided        by wing area or divided by total lifting area)    -   c. low maximum limit maneuver load (n, maximum lift divided by        gross weight)    -   d. very low limit dynamic pressure (½ times air density times        (airspeed))    -   e. very low maximum landing descent rate.

Such constraints render such designs useless for year-round extended,high altitude flight at high altitudes (i.e. 50,000 feet or higher). Itshould also be appreciated that the “cube-square law” (which dictatesthat weight increases geometrically as aircraft dimensions increase)must be considered when contemplating such aircraft. While gossamerdesign can be suitably rugged for small aircraft, aircraft of a sizethat is practical for use in a telecommunications relay network requiresubstantially different construction in order to withstand normaloperating conditions.

The inventors have found, surprisingly, that rigid aircraft can beconstructed that can be sufficiently light for use with solar powerwhile providing rigidity necessary for long duration flight. The currentinvention can use precision molds to manufacture all outer mold line(OML) skins of a composite airframe. This provides the requiredprecision airfoils for high L/D performance at low Re numbers, andadvantageously reduces labor hours required for manufacturing of theaircraft in high quantity production. This is distinct from prior artsolar powered aircraft, which have used open airframes of spars and ribscovered by thin plastic film to minimize weight. Thin-film solar cellscan be conveniently and permanently integrated with the aircraft skinduring manufacturing in such wing skin molds.

Typically, the manufacturing process begins with empty molds that can betreated with a mold-release agent. A thin, clear protective coating isfirst laid in the mold, followed by thin-film solar cells (i.e.photovoltaic cells) arranged in panels. Such solar panels can beflexible (so as to conform to the mold's shape) or provided as rigidpanels that are pre-formed to configure to the mold. Surface wiring isthen applied, followed by the structural layers of the molded compositeskin.

Composite materials suitable for use in structural portions of theaircraft can include a resin (for example, a polyester, a polyurethane,a phenolic, a polyamide, a polyimide, and/or an epoxy) and a fiber orreinforcing component, and can include a catalyst or curing agent.Suitable reinforcing components include carbon (for example, carbonfiber, graphite, and/or carbon nanotubes), aramid (aromatic polyamide)fibers, and/or glass fibers. Fibers used as reinforcing components canbe in the form of tows, yarns, rovings, chopped strands, and/or wovenfiber mats or sheets. Such fibers can be characterized as high modulusfibers or intermediate modulus fibers, depending on their tensilestrength and tensile modulus. High modulus fibers generally have atensile strength ranging from about 700 to 1000 KSI and a tensilemodulus ranging from about 30 MSI to about 45 MSI. Intermediate modulusfibers generally have a tensile strength ranging from about 600 to about700 KSI and a tensile modulus ranging from about 50 to about 85 MSI.Fibers and/or combinations of fibers (for example, high modulus and/orintermediate modulus fibers) can be selected to provide the necessarycombination of stiffness and toughness for the composite material. It iscontemplated that different composite compositions can be utilized indifferent portions of the aircraft. For example, composites utilized atleading edges can differ from those selected for use on trailing edgesand/or control surfaces.

Molded panels of such composite materials can be secured to spars thatprovide internal structure to provide rigid, high precision aerodynamicsurfaces. Spars are preferably constructed of rigid, lightweightcomposite materials, however the use of lightweight metals and alloysfor all or a portion of spar construction is also contemplated.

Use of such composite materials provides an aircraft of the inventiveconcept with sufficient stiffness to adequately resist deformation dueto stresses that can occur during takeoff, landing, and long durationflight at high altitude. Examples of bending stiffness and torsionalstiffness of the wings of aircraft of the inventive concept are shown inFIGS. 9A and 9B, respectively. Such stiffness can be characterized bythe natural frequency of the aircraft wing as a function of the size ofthe wing. In some embodiments of the inventive concept the aircraft caninclude a tail. In such embodiments the natural frequency that can begreater than or equal to 0.65 Hz times (100 ft/wing span, ft)^(1.5),greater than or equal to 1.3 Hz times (100 ft/wing span, ft)^(1.5), orgreater than or equal to 2.6 Hz times (100 ft/wing span, ft)^(1.5). Inother embodiments of the inventive concept the aircraft can have aflying wing design, and thus lack a tail. In such embodiments thenatural frequency that can be greater than or equal to 1.6 Hz times (100ft/wing span, ft)^(1.5), greater than or equal to 3.2 Hz times (100ft/wing span, ft)^(1.5), or greater than or equal to 5 Hz times (100ft/wing span, ft)^(1.5).

An example of an aircraft of the inventive concept is depicted in FIG.4A to 4C. FIG. 4A shows the aircraft with the wing in an essentiallystraight orientation, as would be used at night or at midday. As shownthe aircraft includes a segmented wing 400 that includes a centralsegment 410 and inboard and outboard lateral wing segments (420A, 420B,430A, 430B). As shown, photovoltaic cells are arranged in panels on theupper wing surface. The central segments supports to motor-propellerassemblies (440A, 440B) and a payload 450. As shown, hinges (460A, 460B)couple inboard wing segments (420A, 420B) to outboard wing segments(430A, 430B). Similarly, hinge assemblies (470A, 470B) couple inboardlateral wing segments (420A, 420B) to the central segment 410. In apreferred embodiment the payload can include a telecommunicationstransmitter/receiver. As shown, this embodiment includes a boom mountedtail. It should be appreciated that such an aircraft can include alanding gear, which is not visible in this view.

FIG. 4B shows the same aircraft with the hinges (470A, 470B) positionedto provide a negative dihedral for inboard wing segment 420A and 420B.Similarly, hinges (460A, 460B) are positioned to angle the outboard wingsegments 430A and 430B at a positive dihedral. In this position aportion of the photovoltaic cells towards the horizon, as they would beat lower sun angles (for example, morning or evening). FIG. 4C shows theaircraft with the wing segments positioned at a greater angle than thatdepicted in FIG. 4B.

FIGS. 5A and 5B depict a flying wing embodiment of an aircraft of theinventive concept. FIG. 5A shows an orthogonal view of the aircraft,which has a segmented wing 500 that includes a central segment 510,inboard lateral wing segments (520A, 520B), and outboard lateral wingsegments (530A, 530B). Inboard lateral wing segments (520A, 520B) arejoined to outboard lateral wing segments (530A, 530B) by hingeassemblies (540A, 540B). Similarly, inboard lateral wing segments (520A,520B) are joined to the central segment 510 by hinge assemblies (560A,560B) Photovoltaic cells are arranged in panels on the upper surface ofthe segmented wing. The hinges joining the wing segments permit flexionof the wing in a fashion similar to the embodiment shown in FIGS. 4A to4C. The central segment is shown supporting a pair of motor-propellerassemblies (550A, 550B). FIG. 5B shows a top-down view of the aircraft,showing that wing segments 520A, 530A, 520B, and 530B are angle back inthe direction of travel relative to the central segment 510 in a sweptwing configuration.

A more detailed view of a typical hinge assembly joining an inboard wingsegment to an outboard wing segment is shown in FIG. 5C. The wingsegments are shown with the rigid molded skin rendered transparent. Theforward spar of the inboard lateral wing segment 515 and the forwardspar of the outboard lateral wing segment 535 support a hinge 505B thatlies below a clamshell faring 525. A similar hinge 505A is associatedwith the rearward spars (555, 565) of the wing segments. A linearactuator 545 is also located below the clamshell faring 525. An insetdepicts a portion of the wing when flexed, and shows how the clamshellfaring retain aerodynamic contour on flexion between the inboard andoutboard wing segments. Suitable linear actuators include rotaryelectric motors with suitable gearing, pneumatic actuators, hydraulicactuators, and linear motors.

Similarly, FIG. 5D shows a more detailed view of a hinge assemblyjoining an inboard wing segment to the central segment 523 of a typicalaircraft of the inventive concept. Hinges 517 and 519 associated withfore and aft spars of the central segment 523 and an associated inboardlateral wing segment provide flexion, which is in turn controlled by alinear actuator 521. Suitable linear actuators include rotary electricmotors with suitable gearing, pneumatic actuators, hydraulic actuators,and linear motors.

It should be appreciated that the molded composite nature of the wingstructures of aircraft of the inventive concept can provide for a rigidand robust internal space within the wing segments for placement ofvarious components necessary for aircraft operation. A cross section ofa wing segment of an aircraft of the inventive concept is shown in FIG.7. As shown, a molded composite wing skin 710 has an interior supportedby a fore spar web 740A and its associated spar cap 740B and an aft sparweb 750. The fore spar web can serve to support a battery (or similarpower storage device) 780, electronics packages used in photovoltaiccell and battery power management 760, and a power bus 770. As shown,the wing structure can also include a control surface 730.

The relative lack of structural rigidity in prior art solar poweredaircraft also results in a requirement to distribute weight along thelength of the wing. Such prior art solar powered aircraft, therefore,typically distribute a number of motor and propeller assemblies thatprovide thrust along the length of the wing. The structural rigidity ofaircraft of the inventive concept, however, advantageously permitslocalization of one or more motor and propeller assemblies to a single(for example, central) portion of the aircraft. A reduced number oflarger propellers has the advantage of higher aerodynamic performancedue to higher Reynolds numbers. An example of a typical massdistribution for an aircraft of the inventive concept is shown in FIG.8. For example, in an embodiment of the inventive concept utilizing twoor more motor and propeller assemblies, more than half of these can belocated at a centrally placed portion or segment of the aircraft. In apreferred embodiment the aircraft can utilize two motor and propellerassemblies, both coupled to a central segment of the aircraft andflanked by lateral wing segments that do not carry motor and propellerassemblies. Use of a smaller number of motor and propeller assembliesrepresents a savings in weight and can simplify control. In order toincrease aircraft reliability and further reduce weight, such motor andpropeller assemblies can be arranged so that power is transferred fromthe motor to the propeller directly, without the use of interveninggears (for example, using an iron-less, brushless , ring form electricmotor). In a preferred embodiment the aircraft has two motor-propellerassemblies, which provides redundancy in the event of motor failureand/or propeller damage. In some embodiments propellers can have adiameter ranging from 6 feet to 12 feet or more, and can operate in arange from 50 rpm to 1,500 rpm, 100 rpm to 1,000 rpm, or 175 rpm to 700rpm

It should be appreciated that such a rigid airframe also permits theinclusion of a landing gear on aircraft of the inventive concept. Insome embodiments such landing gear permit the aircraft to launch fromthe ground or other suitably level surface. In other embodiments theaircraft is launched from a catapult, cradle, or similar device and thelanding gear are deployed for landing. In preferred embodiments thelanding gear are retractable. Alternatively, aircraft of the inventiveconcept can include a landing skid.

Characteristics of exemplary airfoils useful in wing segments and/ortail portions of aircraft are shown in Table 1.

TABLE 1 Thickness Incidence Re Airfoil Chord Ratio (degrees) (at 63,000ft) LR0316a 5 16% 3 310,000 LR0314a 3.75 14% 4 230,000 LR0510a 2.5 10%0.5 150,000

It should be appreciated that the aircraft wing is segmented, withindividual segments joined by hinges, and that the airfoils present havelow Reynolds numbers (for example, a Re of 155,000 at the wing tip). Theentire aircraft is designed for low Reynolds number flight at highaltitude with narrow chords and low cruise speeds. In this embodimentthe aircraft has a straight wing measured from the 33% chord stackline.Airfoils were designed for operation between a lift coefficient (CL) of0.5 and 1.2, and different airfoil configurations can be utilized indifferent wing segments. For example, LR03 series airfoils can be usedfor the wing center to the outboard hinge point, and are optimized tobalance low Reynolds number performance with wing strength and stiffnessto weight ratios. LR05 series airfoils can be used for the wingtip toachieve the desired CL range at low Reynolds number.

As shown in FIG. 3, for a given airfoil, drag increases with decreasingReynolds number. This occurs more rapidly below Re of 200,000. The trendshown is for the 16% thickness wing root airfoil operating at Reynoldsnumbers between 100,000 and 3,000,000. To maintain a desired low dragcoefficient at CL=1, thickness can vary with Reynolds number. For thisreason, a 10% thickness airfoil can be used at the low Reynolds numbertip, whereas a 16% thickness airfoil can be used at the root where Re isabove 300,000.

Aircraft of the inventive concept can include control surfaces, whichcan be coupled to actuators that permit such control surfaces to bedeflected to control the aircraft. Such control surfaces can beassociated with a wing, a tail, or both. Examples of control surfacesinclude ailerons and rudders. In some embodiments of the inventiveconcept, the pitch of one or more propeller blades can be controlled; insuch embodiments the propeller blade can be considered as a controlsurface. As moving parts such control surfaces and their associatedactuators are subject to wear, which can lead to mechanical failure andsubsequent loss of the aircraft. While this can be reduced by utilizingmore robust components and/or providing redundant components suchapproaches add weight.

Examples of suitable control surfaces are shown in FIGS. 6A and 6B. FIG.6A shows various positions for a control surface 600A that is coupled toan actuator 610A. As shown, depending on the effective length of theactuator the control surface can be deflected upwards or downwards toprovide rapid changes to the aerodynamic properties of the associatedlifting surface 620. An alternative control surface is shown in FIG. 6B,which shows various positions for a split control surface 600B that iscoupled to an actuator 610B. As shown, both portions of the splitcontrol surface can be moved in concert to provide an upwards ordownwards deflection (similar to the control surface shown in FIG. 6A).As shown, however, such a split control surface can also provide splitdeflection by moving the portions of the split control surface indifferent directions.

Suitable actuators include devices that can provide linear movement, andinclude pneumatic actuators, hydraulic actuators, rotary electric motorsand linear motors. It should be appreciated that split control surfaces(such as those depicted in FIG. 6B) can utilize a pair of actuators,with one actuator associated with each segment and working in concert.Such actuators are preferably lightweight and have a characteristic dutycycle that permits use over a period of years without failure. In apreferred embodiment such flight control actuators show a tightintegration of their internal elements, which can include drive motors,a reticulating ball nut, an output lead screw, and thrust bearings. Themotors are preferably of a brushless, permanent magnet design thatdirectly drives a rotating ball nut and associated thrust bearings. In apreferred embodiment the actuator is a “run dry” unit that does notinclude conventional (e.g. hydrocarbon) lubricants, but rather utilizesself lubricating materials (e.g. tungsten disulfide hardened steel) inconjunction with silicon carbide components (for example, in the ballnut and the thrust bearings) to achieve both a reduction in mass andimproved reliability. In such an embodiment torque resulting from theconversion or rotary to linear motion can be removed directly to wingstructures and/or associated control surfaces through pivot points. Suchan actuator can include a sensor that provides data related to wear (forexample, providing a signal indicating use, a signal indicating responsevs power applied, etc.), and such data can be provided to an onboardcontroller of the aircraft.

Aircraft of the inventive concept achieve safety and reliability in longduration flight, at least in part, by minimizing the use of activecontrol surfaces. Such an aircraft can spend the majority of flightduration using free flight, for example in steady flight and controllingengine speed to execute gradual course changes. In combination withdesign features of the aircraft that enhance stability (for example, theuse of a rigid airframe that resists deformation, swept lifting surfacesthat enhance pitch damping in an all wing configuration, etc.) such freeflight control is adequate to sustain constant flight under slowresponse conditions (e.g. cruising flight at desired altitude and calmweather). Aircraft of the inventive concept can achieve additionalflight stability by positioning of the aircraft's center of gravityrelative to the effective chord of the wing. For example, the aircraft'sweight can be distributed such that its center of gravity has a positivestatic margin of at least 3% of the mean aerodynamic chord. Under rapidresponse conditions (e.g. takeoff, landing, inclement weather, rapidcourse redirections from the onboard controller or an operator),however, aircraft of the inventive concept can be under active flightcontrol (i.e. with control surfaces activated). Aircraft of theinventive concept can be under active flight control (i.e. with at leastone actuator associated with a control surface active) for about 5%,about 2.5%, about 1%, or less than 1% of a flight duration of two monthsor more. Alternatively, an actuator associated with a control surface ofan aircraft of the inventive concept can have a duty cycle that is about5%, about 2.5%, about 1%, or less than 1% of a flight duration of twomonths or more.

An aircraft of the inventive concept can include a controller (forexample, an on-board computer) that can control various aspects ofaircraft function. In a preferred embodiment such a controller caninclude two or more CPUs, which are in communication with sensors thatprovide aircraft telemetry and sensors that monitor the status and/orperformance of various aircraft components (for example, motors,actuators, photovoltaic cells, energy storage devices, wing position,control surface position etc.). Such a controller can, for example,provide adjustment for wing segment position in order to balancecollection of solar power collection against aerodynamic efficiencyand/or energy storage capacity, based on either sensor input or storedinformation related to date and time. Similarly, such a controller canutilize information obtained from sensors (for example, air speedsensors, air pressure sensors, temperature sensors, accelerometers,etc.) to control motor speed, propeller blades pitch and/or activateactuators associated with control surfaces. Such a controller caninclude or be in communication with a database that includes datarelated to use of onboard systems (for example, motors ofmotor-propeller assemblies, actuators coupled to control surfaces,battery charge/discharge cycles, etc.), and can utilize such stored datain deriving instructions for a control maneuver and/or powerdistribution. For example, data indicating that a particular controlsurface actuator has accumulated excessive use or is showing indicationsof wear can be used by the controller to derive instructions that avoidor minimize the use of that actuator. Similarly, data indicating wear orreduced performance of a motor of a motor-propeller assembly can be usedby the controller to derive instructions to reduce the speed of the wornmotor and compensate.

In some embodiments the controller can receive instructions from aremote user and adjust the flight of the aircraft (using free flightcontrol, active control, or both) appropriately. In some embodiments ofthe inventive concept the controller can utilize stored data to predictcomponent failure, and to notify a remote operator regarding such animpending failure. On receiving such a notification the remote operatorcan send instructions to the controller directing it to proceed to arecovery area and land for servicing. Such a controller can beprogrammed to permit flight durations of 1 month, 2 months, 3 months, 4months, 5 months, 6 months, 1 year, 18 months, 2 years, 3 years, 4years, 5 years, or more than 5 years, for example through optimizationof solar energy collection and/or minimization of the duty cycle ofon-board actuators in combination with the use of a rigid airframe thatprovides resistance to non-ideal weather conditions.

Exemplary characteristics of an aircraft of the inventive conceptconfigured as a flying wing are provided in Table 2:

TABLE 2 Wing Reference Total Area 400 ft² Total Span 96 ft Aspect Ratio23 Center Wing Planform Area 59 ft² Half Span 6 ft Stack Line 33% x/cChord 4.92 ft Incidence 0 deg Airfoil LRMII-14 Reynolds Number* −300,000Taper Ratio 1.000 *63,000 ft, 56.3 KTAS, Lower Re at higher altitudeNacelle/Boom Total Length 10.0 ft Maximum Diameter 7.7 in Wetted Area25.3 ft2 Volume 5.4 ft3 Propeller Diameter 8.0 ft RPM Range 225-900 WingTip Area (ea.) 9.0 ft² Length 3.0 ft Root Chord 3.1 ft Tip Chord 2.9 ftSweep −14.0 deg Inboard Wing Planform 188.6 ft² Half Span 21 ft StackLine   33% x/c Leading Edge Sweep 15.0 deg Taper Ratio 0.825 Root MidTip Chord 4.92 4.49 4.06 ft BL 72 198 324 in Incidence 0 −0.25 −0.75 degAirfoil LRMII-14 400INTE7 400INTE3 Reynolds Number* −300,000 −275,000−250,000 Thickness to Chord Ratio 14.0% 13.75% 13.5% Outboard WingPlanform 152.5 ft² Half Span 21 ft Stack Line   33% x/c Leading EdgeSweep 15.0 deg Taper Ratio 0.788 Root Int. 1 Int. 2 Tip Chord 4.06 3.633.42 3.2 ft BL 324 450 513 576 in Incidence −0.75 −2.00 −3.25 −5.00 degAirfoil 400INTE3 400INTE4 400INTE5 400INTE8 Reynolds Number* −250,000−225,000 −213,000 −200,000 Thickness to Chord Ratio 13.5% 12.0% 11%10.0% Control Surfaces Type x/c Deflection Inboard Surface Plain 25%−20° +40° Upper Outboard Surface Split 25% −60° +20° Lower OutboardSurface Split 25% −20° +60°Such an aircraft can have aerodynamic characteristics as shown in FIGS.9A and 9B. Exemplary characteristics of another aircraft of theinventive concept that is configured as a flying wing are shown in Table3:

TABLE 3 Wing Reference Total Area 900 ft² Total Span 150 ft Aspect Ratio25 Center Wing Planform Area 135 ft² Half Span 9 ft Stack Line 33% x/cChord 7.5 ft Incidence 0 deg Airfoil LRMII-16 Reynolds Number* −450,000Taper Ratio 1.000 *63,000 ft, 56.3 KTAS, Lower Re at higher altitudeNacelle/Boom Total Length 15 ft Maximum Diameter 18.8 in Wetted Area56.9 ft2 Volume 18.3 ft3 Propeller Diameter 12.0 ft RPM Range 175-700Wing Tip Area (ea.) 15.3 ft² Length 3.83 ft Root Chord 4.1 ft Tip Chord3.90 ft Sweep −14.0 deg Inboard Wing Planform 438.9 ft² Half Span 33 ftStack Line 33% x/c Leading Edge Sweep 15.0 deg Taper Ratio 0.773 RootMid Tip Chord 7.50 6.65 5.80 ft BL 108 306 504 in Incidence 0 −0.25−0.75 deg Airfoil LRMII-16 INTEB6 INTEB1 Reynolds Number* −450,000−400,000 −350,000 Thickness to Chord Ratio 16.0 15.5 15.0 Outboard WingPlanform 326.7 ft² Half Span 33 ft Stack Line   33% x/c Leading EdgeSweep 15.0 deg Taper Ratio 0.707 Root Int. 1 Int. 2 Tip Chord 5.8 4.954.53 4.1 ft BL 504 702 801 900 in Incidence −0.75 −2.00 −3.25 −5.00 degAirfoil INTEB1 INTEB4 INTEB5 INTEB3 Reynolds Number* −350,000 −300,000−275,000 −250,000 Thickness to Chord Ratio 15.0% 14.0% 13.5% 13.0%Control Surfaces Type x/c Deflection Inboard Surface Plain 25% −20° +40°Upper Outboard Surface Split 25% −60° +20° Lower Outboard Surface Split25% −20° +60°

As noted above, such high altitude, high latitude, and long enduranceaircraft can have considerable utility as a telecommunications relay. Assuch, an aircraft of the inventive concept can include atelecommunications transmitter/receiver. Such a telecommunicationstransmitter/receiver can receive an incoming electromagnetic (forexample, RF or microwave) data signal and rebroadcast it, therebyeffectively extending the range of the originating transmitter. Forexample, a ground-based transmitter or transmitter/receiver canbroadcast an electromagnetic signal that is received by atransmitter/receiver of an aircraft of the inventive concept. Theaircraft's transmitter/receiver can rebroadcast the electromagneticsignal, to be received by another transmitter/receiver (for example, atransmitter/receiver at a different ground station or in a differentaircraft), to establish a communications network that incorporates anaircraft of the inventive concept. Such a network can include aplurality of such aircraft operating in long endurance (i.e. 2 months ormore) flight at high altitude (i.e. greater than or equal to 50,000feet). In some embodiments of the inventive concept such acommunications network can include from 1 to 500 solar powered, longflight duration aircraft. In some embodiments such a communicationsnetwork can include over 500 solar powered, long flight durationaircraft.

It should be apparent to those skilled in the art that many moremodifications besides those already described are possible withoutdeparting from the inventive concepts herein. The inventive subjectmatter, therefore, is not to be restricted except in the spirit of theappended claims. Moreover, in interpreting both the specification andthe claims, all terms should be interpreted in the broadest possiblemanner consistent with the context. In particular, the terms “comprises”and “comprising” should be interpreted as referring to elements,components, or steps in a non-exclusive manner, indicating that thereferenced elements, components, or steps may be present, or utilized,or combined with other elements, components, or steps that are notexpressly referenced. Where the specification claims refers to at leastone of something selected from the group consisting of A, B, C . . . andN, the text should be interpreted as requiring only one element from thegroup, not A plus N, or B plus N, etc.

What is claimed is:
 1. A method of manufacturing a solar poweredaircraft comprising: molding a plurality of wing segments using acomposite material; assembling a wing comprising a plurality of the wingsegments, wherein at least two of the plurality of wing segments arejoined by a hinge and wherein at least one of the at least two segmentscomprises a photovoltaic cell coupled to at least one of at least twosegments and is configured to supply electrical power to the aircraft,wherein the wing has a stiffness that provides a natural frequency thatis greater than or equal to 1.3 Hz times (100 ft/wing span, ft)^(1.5),wherein the hinge is configured to reversibly deflect at least one ofthe at least two segments to a positive or negative dihedral angle whilethe aircraft is in flight, and wherein the aircraft has a center ofgravity having a positive static pitch stability margin of at least 3%of mean aerodynamic chord.
 2. The method of claim 1, wherein theaircraft does not include a tail, and wherein the wing has a stiffnessthat provides a natural frequency that is greater than or equal to 3.2Hz times (100 ft/wing span, ft)^(1.5).
 3. The method of claim 1, whereinthe wing is a swept wing.
 4. The method of claim 1, further comprisingthe step of coupling a plurality of motor-propeller assemblies to acentral segment of the aircraft, and wherein greater than 50% of themotor-propeller assemblies are coupled to the central segment of theaircraft.
 5. The method of claim 4, wherein each of the plurality ofmotor-propeller assemblies comprises an electric motor that is directlycoupled to a propeller.
 6. The method of claim 4, wherein at least oneof the plurality of motor-propeller assemblies comprises a variablepitch propeller.
 7. The method of claim 6, further comprising the stepof incorporating an active control surface that is coupled with anactuator into the aircraft.
 8. The method of claim 7, further comprisingincorporating a controller that is communicatively coupled with at leastone of the plurality of motor-propeller assemblies and with the actuatorinto the aircraft, wherein the controller is configured to optimizeflight reliability of the aircraft through provision of a first commandto the at least one of the plurality of motor-propeller assemblies inslow response conditions and wherein the controller is furtherconfigured to optimize flight safety of the aircraft through provisionof a second command to the actuator in rapid response conditions.
 9. Themethod of claim 8, wherein slow response conditions comprise calmweather conditions.
 10. The method of claim 8, wherein rapid responseconditions comprise inclement weather conditions.
 11. The method ofclaim 8, wherein the controller is configured to maintain flightendurance of the aircraft at an altitude of at least 50,000 feet for atleast 2 winter months at from 40° N latitude to 30° S latitude.
 12. Themethod of claim 8, wherein the controller is configured to maintainflight endurance of the aircraft at an altitude of at least 50,000 feetfor up to 5 years at from 40° N latitude to 30° S latitude.
 13. Themethod of claim 8, wherein the controller is configured to provide anactuator duty cycle of less than 5% during a flight duration of at least2 months.
 14. The method of claim 8, wherein the controller isconfigured to provide an actuator duty cycle of less than 2.5% during aflight duration of at least 2 months.
 15. The method of claim 8, whereinthe controller is configured to provide an actuator duty cycle of lessthan 1% during a flight duration of at least 2 months.
 16. The method ofclaim 8, wherein the controller utilizes free flight control in slowresponse conditions, wherein free flight control utilizes only themotor-propeller assemblies.
 17. The method of claim 1, furthercomprising the step of incorporating a telecommunicationstransmitter/receiver into the aircraft.
 18. The method of claim 1,further comprising the step of incorporating an energy storage deviceselected from the group consisting of a battery, a fuel cell, and acapacitor bank, wherein the energy storage device is electricallycoupled to the photovoltaic cell.
 19. A method of controlling a solarpowered aircraft comprising: providing a solar powered aircraftcomprising a wing, the wing comprising a plurality of the wing segments,wherein at least two of the plurality of wing segments are joined by ahinge and wherein at least one of the at least two segments comprises aphotovoltaic cell coupled to at least one of at least two segments andis configured to supply electrical power to the aircraft, wherein thewing has a stiffness that provides a natural frequency that is greaterthan or equal to 1.3 Hz times (100 ft/wing span, ft)^(1.5), wherein thehinge is configured to reversibly deflect at least one of the at leasttwo segments to a positive or negative dihedral angle while the aircraftis in flight, and wherein the aircraft has a center of gravity having apositive static margin of at least 3% of mean aerodynamic chord;directing the aircraft in a free flight mode for a first portion of aflight duration of at least 2 months, wherein the first portioncomprises at least 95% of the flight duration; and directing theaircraft in an active mode for a second portion of the flight duration,wherein the active mode comprises use of a control surface comprising anactuator, and free flight mode does not include use of any controlsurface of the aircraft.
 20. The method of claim 19, wherein the freeflight mode consists of adjustment of a motor speed.
 21. The method ofclaim 19, wherein the free flight mode is implemented in slow responseconditions.
 22. The method of claim 19, wherein the active mode isimplemented in fast response conditions.
 23. The method of claim 22,wherein fast response conditions are selected from the group consistingof takeoff, landing, inclement weather, and remotely-directed coursechanges.
 24. The method of claim 19, wherein the control surface is anelevator or an aileron.
 25. The method of claim 19, wherein the firstportion is selected to reduce wear of the actuator to provide functionof the actuator for the flight duration.
 26. The method of claim 25,wherein the flight duration is from 2 months to 5 years.
 27. The methodof claim 19, wherein both free flight mode and active mode arecontrolled by a controller located on the aircraft.